Filters are known to provide attenuation of signals having frequencies outside of a particular frequency range and little attenuation to signals having frequencies within the particular frequency range of interest. It is also known that these filters may be fabricated from ceramic materials having one or more resonators formed therein. A ceramic filter may be constructed to provide a lowpass filter, bandpass filter or a highpass filter, for example.
Certain monolithic block ceramic microwave filters, however, also exhibit undesirable passbands at odd harmonic frequencies. This problem typically is present only in higher order modes. This problem is due to the fact that monolithic block filters are made up of quarter wavelength short-circuited transmission lines. As such, resonant transmission lines repeat their characteristics at every half wavelength. At odd quarter-wavelengths, one quarter wavelength and three quarter wavelengths, for example, the electrical impedances of the transmission lines are identical, resulting in unwanted passbands.
The problems presented by these undesired passbands cannot be understated. Oftentimes, the interference will show up in the 2.4-2.7 GHz range at 3*fo the fundamental frequency. At a minimum, there will be unwanted noise in the signal, and if the interference is sufficiently strong, it may result in the telephone call in a cellular system being dropped. This can be both time consuming and annoying for the customer. Additionally, the transmission of harmonics at higher frequencies may create issues for a telecommunications provider which would have to be dealt with by the Federal Communication Commission (FCC).
Consequently, many designers of systems such as cellular telephones need additional attenuation over that provided by traditional monolithic block ceramic filters. To address this problem, designers oftentimes place a second lowpass filter in line to suppress unwanted harmonic responses. This solution, unfortunately, is both expensive and time consuming, and may significantly add to the cost, weight, and part count of a completed product such as a cellular telephone, pager or other electronic signal processing apparatus.
A design which incorporates an integrated harmonic response suppression filter directly into the dielectric ceramic monolithic block could result in a substantial savings in both space and cost.
Another solution to this problem is to add lumped components to the printed circuit board, thereby creating an assembly which properly couples and loads the resonators to eliminate the higher unwanted frequencies. This solution is also expensive, labor intensive, and time consuming.
A ceramic filter with an integrated harmonic response suppression feature which eliminates higher order modes would be considered an improvement in the art.